1. Field of the Invention
The present invention relates to an object information acquiring apparatus and an object information acquiring method.
2. Description of the Related Art
Vigorous research on optical imaging apparatuses that irradiate light from a light source such as a laser onto a biological object, and images the information inside the biological object acquired based on the entered light is in progress in the medical field. An example of an optical image technique is photoacoustic tomography (PAT). In PAT, pulsed light generated from a light source is irradiated onto a biological object, and an acoustic wave generated from biological tissue, which absorbed energy of the pulsed light that propagated and diffused inside the biological object, is detected. This phenomenon of generating the photoacoustic wave is called “photoacoustic effect”, and the acoustic wave generated by the photoacoustic effect is called “photoacoustic wave”.
In many cases, a test segment, such as a tumor, has a higher absorptivity of optical energy than peripheral tissue, and therefore instantaneously expands by absorbing more light than the peripheral tissue. The photoacoustic wave generated during this expansion is detected by an acoustic wave detector, and a received signal is acquired. By mathematically analyzing this received signal, the distribution of sound pressure of the photoacoustic wave generated by the photoacoustic effect inside the object can be imaged (hereafter called “photoacoustic image”).
Based on the photoacoustic image acquired like this, the optical property distribution in the biological object, especially the absorption coefficient distribution, can be acquired. This information can be used for the quantitative measurement of a specific substance in the object, such as glucose and hemoglobin contained in blood. In recent years, vigorous research on photoacoustic imaging apparatuses aimed at imaging blood vessels of a small animal using PAT or applying PAT to the diagnosis of breast cancer or the like is ongoing.
An apparatus for acquiring a photoacoustic image may include a holding plate for holding an object. This is to prevent the object from moving and the position during measurement from changing, and/or making the object thinner by the pressure of the holding plate so as to image the deep portion of the object.
If light is irradiated onto the object held by the holding plate, a photoacoustic wave is generated by the photoacoustic effect not only inside the object, but also in the interface of the holding plate. The photoacoustic wave from the interface of the holding plate is called “interfacial photoacoustic wave”. If this interfacial photoacoustic wave is detected by the acoustic wave detector, noise called “artifact” may be generated.
In concrete terms, if a photoacoustic image is acquired using a received signal including an interfacial photoacoustic wave, an image of a tumor or the like may be hidden by an artifact due to the interfacial photoacoustic wave. The interfacial photoacoustic wave may also be detected by the acoustic wave detector after being multi-reflected inside the holding plate. If a photoacoustic image is acquired using a received signal including a multi-reflected interfacial photoacoustic wave, an image of a tumor or the like may be hidden by an artifact, just like the case of the interfacial photoacoustic wave.
A problem similar to the problem of the interfacial photoacoustic wave in a photoacoustic imaging apparatus also occurs in an ultrasonic diagnostic apparatus. In concrete terms, a transmitted ultrasound repeats multi-reflection inside an interposing object between the acoustic wave detector and the object, such as an acoustic window or an object pressing plate, and appears in an image as a multi-echo artifact.
Japanese Patent Application Laid-Open No. 2000-107177 discloses a method for removing multi-echo generated by an acoustic window constituting an ultrasonic diagnostic apparatus. In Japanese Patent Application Laid-Open No. 2000-107177, multi-echo is removed by subtracting multi-echo extracted by averaging a plurality of received signals from the received signals.
Japanese Patent Application Laid-Open No. 2009-082450 discloses a method for removing a multi-reflected image generated by a plate for pressing an object, which constitutes a medical imaging apparatus to display an ultrasonic diagnostic image. In Japanese Patent Application Laid-Open No. 2009-082450, image data to represent a plurality of ultrasonic images is generated, and a multi-reflected image is extracted from the generated image data. The multi-reflected image is removed by subtracting the extracted multi-reflected image from the image of the object.
Patent Literature 1: Japanese Patent Application Laid-Open No. 2000-107177
Patent Literature 2: Japanese Patent Application Laid-Open No. 2009-082450